In anticipation of an Allied invasion of Japan in 1945, the
Japanese Navy established many bases around Japan with shinyō motorboats. Shinyō, which means "ocean shaker" in Japanese, were two-man
motorboats with 250-kg explosive charges to be used in special attack (suicide)
operations. The shinyō squadron of Seifu Nikaidō, author of this memoir, was
stationed with 25 motorboats for two months at the small fishing village of
Tosashimizu at the southern tip of Kōchi Prefecture. During this time, he faced
death as four orders were given to standby to sortie, but each time the order
was cancelled when no enemy ships appeared. Umi no bohyō: Suijō tokkō
"shinyōtei" no kiroku (Grave markers at sea: Record of sea-based
special attack shinyō boats) focuses on Nikaidō's personal wartime experiences
and also provides a brief general history of shinyō motorboats and other
special attack weapons deployed by the Japanese military.

Nikaidō provides his opinions and personal reflections
throughout the book rather than just reciting the history of his wartime
service. He researched several sources to explain the deployment of shinyō
motorboats in relation to the use of other types of special attack weapons. In
a few places he admits that he does not remember what happened, but this seems
natural since about sixty years passed before he wrote an account of his
wartime experiences. The book has clear organization in three parts: his
wartime service, his postwar feelings and activities related to the war, and
his current opinions about Japan's use of suicide attacks in World War II.

Part 1's wartime experiences begin with Nikaidō's entry into
the Japanese Navy's Preparatory Flight Training Program (called Yokaren in
Japanese) at Tsuchiura Air Base in April 1944 at the age of 16. Although most
of these entering students at Tsuchiura dreamed of becoming pilots, they did
not get the opportunity to do so due to the worsening war situation in which
Japan lacked planes and fuel. On March 15, 1945, about 50 Tsuchiura Yokaren
graduates, including Nikaidō, were transferred to Kawatana Town in Nagasaki
Prefecture to learn to pilot shinyō motorboats on Ōmura Bay. On May 25, when
they graduated from shinyō training, they were assigned to Tosashimizu Base,
still under construction, to be ready to sortie when enemy ships drew near the
shore. Nikaidō's group, named the 132nd Shinyō Special Attack Squadron, arrived
at Tosashimizu on June 26 after the base's completion and did not leave until
August 24, nine days after the emperor's announcement of surrender.

The Japanese Navy's methods many times did not please
Nikaidō, as he mentions in a couple of places the need to get certain things
off his chest. The hanchōs (group leaders) and other officers would frequently
punch students in the mouth with their fists or beat them with a bat for the
slightest infraction. Nikaidō considers this treatment to be worse than that
given to beasts of burden. Later in the book he mentions that even now he,
along with almost all of his classmates, would like to pay back the hanchōs for
the prior violence inflicted upon them. In March 1945, students at Tsuchiura
were asked to step forward one step if they wanted to volunteer for the special
attack corps. Everyone in Nikaidō's group immediately stepped forward with the
knowledge that they would be going to their death. However, this did not
necessarily represent everyone's true feeling but rather the realization that
they would incur the wrath and physical abuse of their hanchō if they did not
do so.

After the end of the war, he returned to his home prefecture
of Hokkaidō and lived like a hermit in a mountain village for about five years.
He then entered the National Police Reserve and later the Ground Self-Defense
Force, from which he retired in 1977 with the rank of commander. Part 2
describes the first annual reunions of the men who served in the 132nd Shinyō
Special Attack Squadron based at Tosashimizu and the 14th Kō Yokaren Class at
Tsuchiura. Typical of long-delayed reunions of men who had served in Japanese special
attack forces, these first reunions occurred more than 30 years after the end
of the war, but they then continued annually thereafter. Nikaidō visited
Tosashimizu three times after the war's end (1973, 1991, and 1993), but he
found the landscape had changed greatly since the war and only located one
person who remembered the shinyō squadron stationed there. On the first trip,
he found the 14 caves where the shinyō motorboats had been hidden. However, by
the last trip only one remained in good condition, but it was being used for
storage of hazardous material.

The book's last part contains the author's thoughts on
certain topics related to Japan's special attack forces. Through the use of
statistics from various sources, he shows that the great majority of special
attack corps members were young men who had been students before entering the
military. Relatively few higher-ranking officers, especially from the military
academies, made special attacks. In several places in the book, Nikaidō states
his belief that the Navy's leaders considered young Yokaren graduates as
"spares" or "expendables." Part 3 of the book also has a
discussion of two events after the emperor's announcement of surrender: the
suicide of Vice Admiral Takijirō Ōnishi, considered to be the originator of
the kamikaze corps, and the last kamikaze attack by Vice Admiral Matome Ugaki,
who led kamikaze attacks from February 1945 until the end of the war. In June
1974, Ōnishi's widow received the highest honor from the emperor in recognition
of her husband's war efforts. Even though Ōnishi's wife soon after contributed
the award in recognition of the sacrifices of many Yokaren graduates, and it is
now at the Yokaren Museum at the former Tsuchiura Air Base, Nikaidō expresses his
doubts as to why such a high award would be bestowed 30 years later to the man
who ordered so many to their deaths.

One fascinating feature of this book is the inclusion of war
song lyrics in several places. These include both songs composed during and
after the war. The book has lyrics to the following songs: "Honoring Our
Comrades" (shinyō memorial song), "Riding a Shinyō," "Young Eagles
Song" (theme song of 1943 movie about Yokaren students), "14th Kō Class
Song" (Yokaren song), "Kaiten Memorial Song," and "Ah, Kamikaze
Special Attack Corps."

This memoir provides many insights into one shinyō pilot's
wartime experiences, especially facts related to the four orders received to
stand by to attack approaching enemy ships, but his descriptions of personal
wartime activities often lack details, probably due to the long time that
passed before putting them in writing. For example, he gives few particulars
regarding his daily activities and his other squadron members during his shinyō
training at Kawatana and his two months stationed at Tosashimizu.